1. Field of the Invention
The present invention relates to an underwater pelletizing assembly, and more particularly to a die plate and gasket for use in an underwater pelletizer.
2. Description of Related Art
Pelletizers are used to process molten thermoplastics into pellets. The pellets may, in turn, be used in other processes for manufacturing various plastic materials or objects.
An underwater pelletizing assembly typically consists of a die plate mounted to an extruding apparatus. Smaller die plates, having a diameter of less than 20 inches, are generally dome or conical shaped. Such smaller die plates are termed dome-shaped die plate assemblies. Larger die plates (in excess of about 20 inches in diameter) are center-mounted, flat die plates having an inner ring of bolt holes, positioned near the center of the plate, and an outer ring of bolt holes, extending around the periphery of the die plate. The subject invention is related to flat die plates.
The extruding apparatus forces molten thermoplastic resin through orifices of the die plate, forming thin polymer strands. The strands are extruded from the die plate into a water bath. More specifically, the proximal side of the die plate may include a plurality of low pressure slots for receiving the molten resin. The molten resin passes from a chamber associated with the slot and into tapered channels extending from the chamber toward an orifice having a diameter of about 0.10 inches and located on the distal surface of the plate. The molten resin is extruded through these orifices, thereby forming the thin strands of polymer resin. The strands are cut by rotating knives positioned adjacent to the distal surface of the die plate. The cutting takes place underwater in the water bath. The cut pellets contact the cooler water and harden, thereby forming thermoplastic pellets of a generally uniform shape and size.
Typically, an underwater pelletizing assembly is configured so that a constant stream of water passes over the distal surface of the die. The water must be cool enough to permit solidification of the extruded polymer at an acceptable rate. Specifically, the pellets should solidify before being permitted to deform as a result of contact with adjacent pellets or the sides of the water container or conduit. The hardened pellets are transported from the die face by the constant water stream. The cut pellets are removed from the water stream by a filtration apparatus. Once removed from the water stream, the hardened pellets may be dried using a blower, heater, or similar drying apparatus.
To form pellets of a specific size and shape, the orifices of the extrusion die must remain free and clear of solidified polymer material. Occlusion of portions of the orifices by solidified polymer material causes formation of irregular shaped pellets. If the polymer resin does not harden until it is removed from the die face, then the possibility of occlusion of the channels is substantially reduced. However, in that case, the formed pellets may not solidify fast enough. The pellets may contact one another or deform against the sides of the water bath.
In many underwater pelletizing systems, a heating device is used to heat the die plate to ensure that thermoplastic resin passing therethrough does not solidify until after it is expelled from the orifices. For example, a die plate may include electrical heating coils extending through the plate structure for selectively providing heat to the plate surface. Alternatively, the die plate may be exposed to a heating fluid such as hot oil or steam to maintain a desired plate temperature.
Several problems result from exposing a heated die plate to a constant stream of cooling water. Most significantly, water flowing across the die plate may leak into bolt holes causing corrosion and damage to the bolt holes and bolts. Sealing structures, such as gaskets, are used to prevent such degradation of the bolts and bolt holes.
In addition, water passing over the face of a heated die plate effectively dissipates heat from the die plate, unnecessarily cooling the die plate and heating the circulating water. Therefore, an insulating material is often placed between the die plate and water stream to prevent unnecessary heat loss to the water stream. By reducing heat loss to the water stream, the energy required to heat the die plate to the sufficient temperature, to prevent hardening of polymer resin in the orifices of the die plate, is effectively reduced.
With reference to FIGS. 1-3, a die plate 10 for an underwater pelletizing assembly is depicted, as is known in the prior art. Such die plates 10 are commercially available from a number of sources, including Kennametal Inc. of Latrobe, Pa. The die plate 10 is a center-mounted flat die plate having of an inner ring 12 of through holes 14 and an outer ring 16 of through holes 15. Fasteners 18, such as bolts, extending through the through holes 14, 15 mount the die plate 10 to other elements of the assembly, such as the extruding apparatus. The die plate 10 is configured to be heated by a fluid heating substance, such as steam or oil.
To prevent water from entering the through holes 14, 15, gaskets are placed over the through holes 14, 15 to form a seal therewith. Gaskets suitable for use with die plates are also manufactured by Kennametal Inc. Gaskets suitable for use with Kennametal die plates are also available from a number of third party manufacturers. As shown in FIG. 1, two separate gaskets are provided. An outer gasket 22 covers the outer ring 16 of through holes 15. An inner gasket 24 covers the inner ring 12 of through holes 14.
With continued reference to FIGS. 1-3, the outer gasket 22 is a ring-shaped gasket. The inner gasket 24 is a disc-shaped gasket covering the inner ring 12 of through holes 14, as well as the central portion of the die plate 10. Each gasket 22, 24 is covered by a metallic retainer plate, namely outer retainer plate 27 and inner retainer plate 26, that are similar in shape to the respective gaskets 22, 24. The gaskets 22, 24 and retainer plates 26, 27 are attached to the die plate 10 by retainer screws 52 inserted through corresponding retainer holes 54. The gaskets 22, 24 provide effective seals for the through holes 14, 15. In addition, the gaskets 22, 24 provide effective insulation between the water stream and the distal face of the die plate 10. Particularly, the gaskets 22, 24 ensure that the water stream is not in direct contact with the plate 10 or with the fasteners 18.
Gaskets 22, 24 are commonly constructed from elastomeric materials, such as Aflas (a copolymer of tetrafluoroethylene (TFE) and propylene (P)) and Garlock (a polytetrafluoroethylene (PTFE) containing inorganic microsphere additives). The gaskets 22, 24 may consist of a single layer of an elastomeric material or may include multiple layers laminated together. The multiple layers may be formed from different elastomeric materials to obtain different insulating or sealing characteristics. Gaskets 22, 24 maintain structural integrity when exposed to temperatures up to about 440° F. As long as the continuous water stream is provided to dissipate heat from the distal surface of the die plate 10, the gaskets 22, 24 are not exposed to temperatures in excess of the 440° F. upper boundary. However, if the water stream is stopped prior to the die plate 10 cooling down, then, since the gasket 22, 24 is no longer being cooled by the flow of water, but is still being heated, the gasket 22, 24 may overheat to temperatures in excess of 500° F. Exposure to such elevated temperatures causes the gasket 22, 24 to warp and form gaps between the gasket 22, 24 and die plate 10. Water leaks through the gaps and collects between the gasket 24 and surface of the die plate 10. When the heating mechanism supplying heat to the gasket is turned on again, the water is rapidly converted to steam. While the gaps are large enough to allow the liquid water to seep in, they are not large enough to permit rapidly expanding steam to exit. As a result, steam may become “trapped” under the disc-shaped inner gasket 24. The trapped steam may exert substantial pressure on the gaskets 22, 24, retainer plates 26, 27, and retainer screws 52 connecting the inner gasket 24 to the die plate 10. Such pressure may be sufficient to separate the inner gasket 24 and inner retainer plate 26 from the die plate 10 and irreparably damage the inner gasket 24 and inner retainer plate 26.
In view of the difficulties associated with die plates 10 and the inner gasket 24 of the prior art, there is a need for a die plate 10 and inner gasket 24 for an underwater pelletizer that effectively seals through holes 14 of the inner ring 12 of a center-mounted die plate 10. The inner gasket 24 and outer gasket 22 should also effectively insulate the heated die plate 10 from the continuous cool water stream, to prevent heat loss from the die plate 10 and to improve energy efficiency of the die plate 10. Additionally, the die plate 10 and inner gasket 24 should be configured to avoid trapping steam between the die plate 10 and inner gasket 24 to prevent gasket failure when water between the die plate and gasket is converted to steam. Finally, it would be beneficial for the die plate 10 to limit the types of gaskets that may be applied thereto. Specifically, gaskets which tend to excessively trap steam should not be able to be used with the die plate and underwater pelletizing assembly.